Technical Program

The technical program for this research effort will comprise statistically designed tests to generate data about critical parametric effects on corrosion of steels followed by data analysis and synthesis to generate corrosion prediction guidelines. Data generated will include quantified corrosion rate information, localized corrosion tendencies and scale stability. The technical program has been classified into six tasks described in this section.

Task 1 - Steel Selection and Specimen Preparation

Alloy metallurgy is known to be one of the most significant parameters in mitigating corrosion in steels. Small additions of Chromium and Molybdenum can significantly alter steel corrosion behavior. The focus of this task is to select grades of steels to be used for lab testing based on a careful analysis of potential metallurgical effects. The underlying idea is that selecting appropriate grades of steels will lead to obtaining an understanding of how metallurgy and processing affect corrosion resistance/damage. Typical compositional variables in steel selection are:

Alloy Composition: Chromium (0 to 1.5 percent), Nickel (0 to 2.5 percent), Molybdenum (0 to 1 percent), Copper (0 to 0.4 percent) etc.

Metallurgical Processing: Hot Rolled, Normalized, Quenched & tempered, Thermomechanically controlled processing (TMCP), Welded, Welded and stress relieved.

Carbon and low alloy steels will be selected from commercially available materials where possible. These will include Pipeline materials including both conventional and 0.5 Cr steels, Oil Country Tubular Goods (OCTG) including both C-Mn and C-Cr-Mo (Modified AISI 4100 series steels) and Standard wrought AISI steels used in downhole and surface equipment.

Specimen Design. Appropriate specimen design is essential for obtaining meaningful data from lab evaluations. The corrosion tests in this program will be conducted using a combination of electro-chemical and dynamic closed-loop flow tests and will include two types of test specimens:

• Flow through electrode (FTE) coupons. This coupon design is shown in Figure 2. They involve a tubular geometry and allow for the corrosive environment to flow through the I.D. of the specimen so that the effects of flow and velocity can be determined.
• Impingement electrode (IE) coupons. This coupon design is shown in Figure 3. It consists of a ring of the test material embedded into a nonconductive mount. This allows for tests to be conducted under constant rheology under conditions of liquid phase impingement.

Figure 2 - Flow-through electrode coupon	Figure 3 - Impingement electrode coupons

Task 2- Evaluation of Steels in Carbon Dioxide/Hydrogen Sulfide Environments

The mechanism of CO₂ corrosion from a thermodynamic standpoint has been well understood and has been the focus of many other modeling efforts. A commonly accepted mechanism of CO₂ corrosion involves anodic dissolution of Iron (a pH dependent mechanism) as well as the cathodic reduction of undissociated carbonic acid. However, when H₂S is present in produced waters, H₂S related corrosion and cracking dominates in most scenarios but for those with very low levels of H₂S (CO₂ / H₂S ratio more than 500). While material evaluation for H₂S is one of the biggest concerns in industrial corrosion and cracking, there is a need for data generation to model H₂S corrosion and scaling effects. In this program, tests will be conducted in CO₂ environments and in CO₂ / H₂S environments (as it occurs in reality) to investigate H₂S-related effects in relation to CO₂ corrosion.

The first set of tests in this task will be in pure CO₂ environments with the following ranges:

• Carbon Dioxide:  15 to 200 psia
• Chlorides:            0 to 150,000 ppm
• Flow Rate:           5 to 25 feet per second
• pH:                      3.0 to 5.5
• Temperature:        80 to 350 F

Tests will monitor corrosion rate versus time with increasing flow rate. A total of 20 flow loop tests are anticipated in this sub-task involving the exposure of over 100 FTE coupons and 20 IE coupons. The test matrix will be based on a statistical 1/4 to 1/2 factorial design with mid-point replication. The flow loop to be utilized in this program is InterCorr's proprietary Multiphase Autoclave Pipeline Simulator (MAPS^TM). It consists of a five liter Alloy C-276 lined autoclave and provides for flow simulation with an Alloy C-276 external loop and pump system.

In these tests, the FTE and IE coupons will be monitored using the linear polarization resistance technique per ASTM G59. An external high pressure Ag/AgCl reference electrode will be utilized in combination with the C-276 system as the counter electrode for making potential measurements. Electrochemical impedance spectroscopy (EIS) will be utilized in selected tests to examine fundamental properties of corrosion film formation. The test duration will be up to four days depending on the severity of the environment.

Upon completion of the tests, the FTE and IE coupons will be evaluated to determine both scale weight and thickness, average mass loss rate and maximum localized corrosion rate by techniques given in ASTM G1 and G46. Particular attention will be given to conditions that appear to promote localized corrosion versus general corrosion as measured by the ratio of localized to general attack. Conditions that produce ratios of <2, 2-5, 5-10 and >10 will be identified.

The second phase of this task involves testing using InterCorr's MAPS^TM system with the addition of Hydrogen sulfide (0.05 - 200 psia) to the operating conditions specified above. The focus of this phase of testing will be to analyze manifestation of H₂S corrosion in terms of formation and stability of FeS and its variants such as mackinawite and pyrhotite. FeS scale stability as a function of temperature and pH will also be studied. Further surface properties of FeS scaling in terms of porosity and propensity for acceleration of localized attack in chloride-laden environments will be documented. This task will generate data that can provide an understanding of critical areas of H₂S-related corrosion that have hitherto received little or no attention.

Task 3 - Analysis of Hydrocarbon and Inhibitor Film formation on selected Steels

The goal of this task is to identify the effects of mitigating factors such as presence of liquid hydrocarbons and chemical inhibitor compounds on the severity of corrosion of steel in selected carbon dioxide and hydrogen sulfide containing environments. Initially, four base hydrocarbon liquids will be selected from hydrocarbons of various API gravity and maturation.

This task will generate data to support determination of those constituents in the hydrocarbon phase that promote creation of an oil-wet metal surface which can inhibit corrosion. Further, relationship between oil wettability, persistence of oil phase and water cuts in sour systems will be examined. Data to help identify relative wettability of the oil phase with respect to the water phase will also be generated.

Flow loop tests will be conducted in two types of environments (CO₂, CO₂ / H₂S) at two test temperatures over a range of flow rates with different combinations of hydro-carbon and inhibitor types. These tests will facilitate determination of hydro-carbon and inhibitor film characteristics as a function of chemical species present, temperature and flow rates.

Two types of inhibitors will be utilized for these tests; one for the CO₂ environments and the other for environments containing CO₂ and H₂S. They will be selected based on typical chemistry for inhibitor compounds. This task will specifically indicate any difference in mitigating effects of the liquid hydrocarbon on the corrosion rate of various steels in CO₂ and CO₂ / H₂S environments. Magnitude of the liquid hydrocarbon protection versus exposure conditions as a function of hydrocarbon type will be determined.

Task 4 - Generation of field data for select parametric combinations

Field testing of specific selected materials under differing combinations of CO₂ / H₂S partial pressures, hydrocarbon chemistry and inhibitors will be conducted in this program to generate a basis to relate lab results to actual field observations. Field data will be analyzed with a view to verify lab studies with field corrosion behavior. Data generated here will also be used to calibrate the prediction model developed in this program so as to provide a realistic tool for corrosion prediction and assessment.

Figure 4 - Hydrocarbon component classifications

Task 5 - Data interpretation and development of corrosion prediction guidelines

The results of the program will be analyzed in terms of general and localized corrosion rates and corrosion scaling tendencies of various steels as a function of metallurgical variables, environmental conditions and flow characteristics. Three most significant outcomes from this research effort will be,

1. Modeling of H₂S related corrosion and scaling
2. Determination of conditions that promote oil wettability as a function of hydro-carbon composition
3. Prediction and assessment guidelines based on lab data and field verification

A computer database for all the data generated in this program will be developed. Data analysis will involve determination of relationships and trends based on multi-variant statistical methods including regression-analysis, numerical interpolation and curve-fitting. This task will produce guidelines and rules that can be used to identify conditions of significant corrosive severity as well as presence of mitigating factors. The analysis will also assist in predicting corrosion-product scale stability and hydro-carbon/inhibitor film persistence.

Task 6 - Incorporation of data and guidelines into Prediction tool

The use of the statistically designed test matrices will facilitate development of algorithmic relationships between environmental parameters and metallurgical parameters and provide the bases for capturing parametric interactions. These results will be used to develop rules which will be incorporated into InterCorr's PREDICT^TM software to produce a computer-based tool for accurately assessing and predicting corrosion rates.

The sponsors will receive a preliminary hard copy version of the envisioned new rules for review and comment. Following receipt of these comments, InterCorr will prepare a single prototype system including the new rules for data testing by the sponsor representatives. The sponsor comments from this prototype software will be evaluated and an enhanced version of the PREDICT^TM (single-user) program will be provided at the completion of the program.